A light fixture that emits light in only a narrow range of directions is said to provide normalized output. Such light sources are often desirable. Because most light sources in common use produce an undirected output, some technique must be used to normalize the light they emit. One common method of normalizing the output of a light source is the use of a parabolic reflector. Such a reflector will produce a collimated beam of light from a point source.
The problem becomes more difficult when the light source is an extended linear light source, such as a common fluorescent lighting tube. Light from a fluorescent tube may be partially collimated by a reflector which is parabolic along a cross-section running perpendicular to the major axis of the tube. This technique will normalize the direction of the light that is emitted perpendicular to the tube, but not the light that is emitted in other directions.
Another technique that is commonly used with extended linear light sources involves positioning louvers in front of the light source. Such louvers allow only light traveling in the normalized direction to pass.
Both of these techniques suffer from common disadvantages. One such disadvantage is that light fixtures utilizing them are necessarily bulky. Parabolic reflectors, by their very nature, must be deep in relation to the size of the aperture. This is particularly true if the parabolic reflector is to normalize wide angle light. Similarly, if the louvers are to have a significant normalizing effect they must be reasonably deep.
A second disadvantage of both of these techniques particularly when a fluorescent tube is used as a light source for a light fixture which is to be used as an area source, is that both will produce brighter areas, commonly known as "hot spots". Hot spots normally arise in the portions of the light fixture closest to the light source. If such hot spots are to be avoided, other techniques must be used.